Modern gravure printing is done principally from images etched in cylinders which are used on web presses and is generally referred to as rotogravure, although similarly etched flat plates can be employed as well. On sheet-fed presses the printing element is generally a thin copper plate wrapped around the cylinder. Preparation of the printing surface is essentially the same for both cylinders and plates.
For monochrome printing by conventional gravure, bichromate-sensitized gelatin coated carbon tissue or transfer film is first contact printed through a continuous-tone positive and then given a second exposure in contact with a screen consisting of transparent lines and opaque square dots, 150 or 175 to the inch. The ratio of line to dot width is usually 1:3. The exposed carbon tissue or transfer film is then moistened and squeegeed into contact with the clean copper surface. Warm water is applied and the paper of the carbon tissue or backing of the film is peeled off. The gelatin which is transferred to the copper surface is then further developed with warm water to produce a gelatin relief resist. Etching is done with 35-43 percent ferric chloride solution. The solution etches the copper to different depths depending on the thickness of the gelatin resist in the different tone areas. The areas corresponding to the screen lines remain unetched and provide "lands" to support a doctor blade in printing. For long runs, the etched cylinder or plate is chromium-plated to resist wear.
For multicolored printing the principal process consists of making both a special halftone positive with a lateral dot formation which is similar to conventional gravure in the shadows but with varying dot sizes and a continuous-tone positive for each color. These are contact-printed successively, in register, onto a sheet of carbon tissue, and the gelatin is transferred to the copper cylinder. Development and etching are essentially the same as for monochrome cylinders. The printing surface thus consists of disconnected ink cells of varying size and depth corresponding to the tone values desired.
While gravure printing is a preferred method of printing by reason of its superior quality, the methods for preparing gravure printing members are relatively expensive, time consuming and thus generally uneconomical for short runs. In addition, the gravure plates can be used only for one image and the image cannot be removed and the plate reused. While gravure cylinders can have the old image stripped off and the cylinder replated for reuse, this is an expensive and time consuming process. Thus it would be desirable if a process could be developed for rapidly and inexpensively forming gravure printing members particularly well suited for short runs.